I have heard it is safe as long as there is no formaldehyde in the hair color -- apparently this used to be a more common ingredient. Most dyes/bleaches do not contain this ingredient but you should ask the hair stylist to check or show you the package. I didn't highlight or lowlight my hair until after the 1st trimester either (it looked a wreck for awhile!).
Maybe you could consider just getting some lowlights. (Like highlights, except they paint in a darker color instead of a bleach, when the put it in the foil.) I highlighted my hair when I was pregnant, with no problems.
colouring your hair is safe as long as there is no henna in the dye as it can affect the baby as well as your self, I normally go black but am having to use brown instead also some hair dressers will dye your hair while others won't. Just be careful and don't dye your hair to much I dye mine every 6-7 weeks now were as before I done it every 4-5 weeks